In recent years, development has been advanced for next-generation negative electrode active materials having a charge and discharge capacity substantially exceeding the theoretical capacity of carbon materials as negative electrode active materials for lithium-ion secondary batteries. For example, materials containing metals capable of forming alloys with lithium (Li) such as silicon (Si) and tin (Sn) are highly anticipated.
These active materials such as Si and Sn have poor electron conductivity. When the electrical conductivity of the negative electrode is poor, the internal resistance of the electrode increases, which causes the cycle characteristics to be diminished. Therefore, it is typical to add a carbon material such as graphite or carbon black to the active material layer as a conductive material. However, it has become clear that even if a carbon material is used as a conductive material, the resistance will no longer decrease once a certain amount has been added.
In particular, when Si, Sn, or the like is used as an active material, it is difficult to favorably maintain the adhesive state between the current collector and the active material since these materials undergo large changes in volume in response to the absorption and release of Li at the time of charge and discharge. In addition, these materials have an extremely large volume change rate in response to the insertion and desorption of Li, and the active material particles are pulverized or desorbed as result of repeated expansion and contraction due to the charge-discharge cycle, which leads to the drawback that cycle deterioration is very large.
A copper foil disclosed in Patent Document 1 (Japanese Patent No. 4583149), which was developed for a flexible printed circuit board (FPC) to be laminated with a film (polymer material), can withstand the heat treatment at 180° C. for 1 hour required for lamination with a film and has high tensile strength.
However, such a copper foil for an FPC normally must be able to withstand heat treatment at 350° C. for 1 hour as a current collector for a battery. When the copper foil is exposed to such temperatures, the crystals become coarse, and the tensile strength cannot be maintained at 300 MPa or higher after heating, so the copper foil cannot be utilized as a current collector for a secondary battery. The reason for this is that an active material composition prepared in the form of a paste by adding a solvent or the like to a mixture of an active material, a conductive material, and a binder is applied to the surface of the current collector for a lithium-ion secondary battery, and a negative electrode of the lithium-ion secondary battery is formed via a drying process, but heat treatment at 350° C. for 1 hour is normally required for the drying process. When the copper foil for an FPC described above is used directly, the crystals of the copper foil become coarse, and the tensile strength after heating cannot be maintained at 300 MPa or higher. Therefore, the copper foil cannot withstand the expansion and contraction caused by the charge-discharge cycle of the active material, and there is a possibility that the copper foil may rupture as a result.
An active material using a pitch coke material is disclosed in Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2004-079523A). It has become clear from research in recent years that the active material disclosed in this document undergoes smaller changes in volume in response to the absorption and release of Li at the time of charge and discharge than Si or Sn alloys. However, the surface roughness of the copper foil is very low, so the surface of the material is smooth. Therefore, when such a copper foil is applied to an active material using a pitch coke material, the changes in the volume of the copper foil in response to the absorption and release of Li at the time of charge and discharge are larger than those of the coke material, which may cause the detachment of the copper foil and the active material and the reduction of the contact area with the active material so that charging and discharging cannot be realized.
A copper foil disclosed in Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2010-282959A) addresses this issue by roughening both sides of the copper foil in order to enhance the adhesiveness between the active material and the copper foil. The problem of the detachment of the active material and the copper foil can be solved by the roughening of the copper foil surface. However, no consideration is given to the difference in the degrees of roughness on the front and back sides of the copper foil. In particular, since an active material comprising a Si or Sn alloy has a very small particle size, it is not always possible to uniformly apply the active material to both the front and back sides. Therefore, the copper foil may deform and develop wrinkles as a result of expansion and contraction of the active material due to charging and discharging, which leads to the risk that the material may not be usable as a battery.
It is disclosed in Non-Patent Document 1 (Lakshmanan et al., “The effect of chloride ion in the electrowinning of copper”, Journal of Applied Electrochemistry 7 (1977) 81-90) that the state of the surface of a copper foil is dependent on current density. That is, it is disclosed that, in the foil production process, a smooth surface is obtained by limiting the current density to a lower level at a chlorine ion concentration of 0 ppm.
In addition, Non-Patent Document 2 (Anderson et al., “Tensile properties of acid copper electrodeposits”, Journal of Applied Electrochemistry 15 (1985) 631-637) discloses that, as disclosed in FIG. 7, although the initial maximum tensile strength is high when the chlorine ion concentration in a copper sulfate plating bath is 0 ppm, the elongation is low. When the chlorine ion concentration is 5 ppm, the initial maximum tensile strength decreases dramatically, and the elongation increases dramatically with inverse proportion to the maximum tensile strength. This suggests that when chlorine ions are added at a concentration of at least 10 ppm, the maximum tensile strength and the elongation rate demonstrate gradual changes with inverse proportion to one another.
With the techniques disclosed in the documents described above, it is possible to change the surface roughness by controlling the current density. In order to obtain an electrodeposited copper foil with elongation that is not diminished substantially even if the maximum tensile strength is increased, it is preferable to produce the foil with a chlorine ion concentration of at least 5 ppm, which can be read as a suggestion that it is preferable to produce the foil with a chlorine ion concentration of at least 10 ppm. However, these documents do not disclose the detailed techniques for obtaining a copper foil with a tensile strength of at least 300 MPa after heating at 350° C. for 1 hour and an elongation rate of at least 3.0% after heating at 350° C. for 1 hour.